Control of a multi-carrier power amplifier

ABSTRACT

A method of operating a Time Division Multiple Access (TDMA) communication system is disclosed. The system includes a multi-carrier power amplifier. The method includes the steps of logging call activity on each timeslot of each carrier associated with the multi-carrier power amplifier; logging total transmit power of the multi-carrier power amplifier for each timeslot; and logging the data rate of any data calls being carried by the multi-carrier power amplifier. Transmission power is made available for a new call by reducing the transmission power of a data call.

This invention relates to a method and apparatus for dimensioning andcontrolling a multi-carrier power amplifier. It finds particular, butnot exclusive use in a Base Transceiver Station (BTS) in a GSM basedcommunication system adapted to use the EDGE data transmission standard.

Traditionally, GSM BTSs have employed single-carrier power amplificationschemes. That is, a single amplifier chain was provided for each GSMcarrier signal. Each GSM carrier signal comprises 8 time slots perframe, and is thus theoretically capable of supporting 8 conversationsor data connections simultaneously.

Typically, a BTS comprises several transceiver units, each one having adedicated power amplifier (PA). FIG. 1 shows a BTS configurationaccording to the prior art. In this example, eight transceiver units(TRX) are provided 10 a-10 h. Each transceiver unit has an associatedpower amplifier (PA) 20 a-20 h. The Power Amplifier is responsible forboosting the output power of the TRX to a suitable level fortransmission. However, the resultant signals from each of the PAs haveto be combined in order to route them to a common transmission antenna.This requires the use of a high power combiner 30, which has thedrawback that a considerable amount of the input power is dissipated inthe combining process. Typically, 3.2 dB is dissipated in heat per 2-waycombine in a hybrid combiner. As the output of eight TRXs needcombining, there are 3 combining stages needed, resulting in a loss ofnearly 10 dB, or 90% of the amplified signal. This places constraints onthe BTS in the fields of power regulation and thermal design.

The configuration is actually more complicated due to the provisionsmade for the receive path from the antenna, but we are only concernedwith the transmission path here.

It is now possible to implement PAs which are capable of amplifying morethan a single carrier signal. These are known as Multi-Carrier PowerAmplifiers (MCPAs) or Multi-Carrier Linear Power Amplifiers (MCLPAs).Such a configuration is shown in FIG. 2. The TRXs 10 a-10 h areidentical to those shown in FIG. 1, and each supports a single GSMcarrier as before. The outputs from the TRXs are next combined in a lowpower combiner 50. The relative losses in this are still of the order of10 dB in total, but as the input power to the combiner is considerablylower, the absolute power loss is much lower.

The output of the combiner 50 is next fed into the input of the MCPA 60.The MCPA is a wideband linear amplifier which, in this instance, iscapable of amplifying the outputs of all eight TRXs simultaneously,before transmitting the signals via antenna 40.

One of the problems which has thus far held back deployment of MCPAs isthe linearity which is required by the GSM specifications. GSMspecification 05.05 Section 4.2.1, “Spectrum due to the modulation ofwideband noise”, particularly sets the limits on the acceptable levelsof noise products due to non-linearity effects in the PA. Only recentlyhas it been possible to implement MCPAs which meet all the necessarycriteria laid out in the GSM specifications.

Defining the transmit power output requirement for a single carrier PA(SCPA) is straightforward. The SCPA is capable of supporting up to eightsimultaneous connections—one on each of the timeslots which make up aGSM frame. Each timeslot is processed in turn, and so the maximum outputpower required form the SCPA is equivalent to the maximum power calledfor in any one of the timeslots. Defining the power output requirementfor an MCPA can be more problematic. An assumption made in specifyingthe power output requirement for an MCPA is that if it is operatingsubstantially linearly, then the total output power on a given timeslotis given by the sum of the individual powers of each carrier.

$P_{TOT} = {\sum\limits_{0}^{n}\; P_{n}}$where n=number of GSM carriers being amplified, which is eight in theexample cited in FIG. 2.

A simple means of defining the maximum power required would be to assumethat each individual carrier was operating at its maximum level, so thatthe total power output required is equal to the sum of each individualmaximum power requirement, orP _(TOT) =n·P _(MAX)

While this solution will produce an MCPA which will support each GSMcarrier signal successfully under all conditions, it will be greatlyover-specified, and will consume excessive amounts of energy. Forexample, linear PAs tend to be only 5-7% efficient, or to put it anotherway, 93-95% of the energy supplied is dissipated as heat. Thisinefficiency is due to the mode in which the amplifiers need to operatein order to meet the linearity specifications. The net result is thateven if the transmitter is required to provide a relatively low amountof output RF power, or even no output at all, the amplifier remainsbiased in such a way that large amounts of heat have to be dissipated.For example, assuming that an MCPA supports eight 1 W carriers, then itmust be designed to dissipate between 100 W and 150 W of heat energy.

Not only do such requirements impose difficult design constraints onBTSs in terms of heat dissipation, but component failures increase withsuch increases in temperature, posing reliability problems. The addedcharges for electricity also become significant when applied across anentire cellular network.

According to a first aspect of the present invention, there is provideda method of operating a Time Division Multiple Access (TDMA)communication system comprising a multi-carrier power amplifier (MCPA),comprising the steps of: logging call activity on each timeslot of eachcarrier associated with the multi-carrier power amplifier; logging totaltransmit power of the multi-carrier power amplifier for each timeslot;and logging the data rate of any data calls being carried by themulti-carrier power amplifier, wherein transmission power is madeavailable for a new call by reducing the transmission power of a datacall.

According to a second aspect of the present invention there is provideda communication system comprising: a multi-carrier power amplifier; anda database comprising: information concerning call activity on eachtimeslot of each carrier associated with the multi-carrier poweramplifier; information concerning total transmission power of themulti-carrier power amplifier for each timeslot; and informationconcerning the data rate of any data calls supported by themulti-carrier power amplifier wherein the multi-carrier power amplifieris arranged to reduce the transmission power associated with a data callin order to make transmission power available for a new call.

Advantageously, the method and system may be applied to anycommunication system adapted to use the EDGE standard. EDGE iseffectively an overlay which may be used with TDMA based communicationsystems such as GSM or IS-136. It uses substantially the same hardwareand protocols, but uses a different modulation scheme to achieve higherdata rates.

Advantageously, the realisation that the data rate on a particular callcan be reduced, to free up transmission power for the new call, meansthat the new call can be accommodated at the expense of a potentiallysmall, and possibly temporary, drop in the data rate of a single call.

Preferably, the assignment of timeslots to calls proceeds such thatthere is minimal variation in transmit power from one timeslot to thenext. This ensures that should there be a demand for increased transmitpower for any one call, then sufficient transmit power capacity shouldbe available across all timeslots.

Preferably, if a call data rate has to be reduced to accommodate a newcall, then the data call currently having the highest data rate willhave its rate reduced, via reduced transmit power, before other calls.As the call parameters such as transmit power and data rate arecontinuously monitored, this test is continuously re-evaluated.

In an advantageous development, each call may have an assigned prioritylevel. This may be due to the particular charging tariff to which asubscriber belongs, or it may be that a user can elect to pay a premiumfee on a call by call basis to ensure that a high speed data connectionis maintained for that call. Other scenarios exist where calls can beprioritised. Call priority level can be used to determine which of thecurrently supported calls can have its data rate reduced, with lowerpriority calls being affected first. The incoming call which isattempting call set up may also have a priority level which willinfluence how timeslots are allocated to it.

Calls may have associated with them a minimum data rate, below whichthey will not drop. In a preferred embodiment where EDGE is used toachieve higher data rates, then the minimum data rate corresponds to thedata rate which could be achieved if the modulation scheme of thenetwork underlying the EDGE standard were used. In the case of a GSMnetwork, GMSK would be used to modulate the data. Using this modulationscheme, the data rate would be 9.6 Kbit/s.

In a case where the MCPA is not operating at maximum transmit power oneach timeslot, but it has insufficient free power available on any onetimeslot to be able to support a new call, then it is possible toreallocate the current timeslot assignments in such a way that whatspare transmit power is available is reallocated to the same timeslot.This enables a carrier using that timeslot to support the new call.

For a better understanding of the present invention, and to understandhow the same may be brought into effect, the invention will now bedescribed, by way of example only, with reference to the appendeddrawings in which:

FIG. 1 shows some elements of the transmitter chain in a BTS utilisingsingle carrier power amplifiers according to the prior art;

FIG. 2 shows some elements of the transmitter chain in a BTS utilising amulti-carrier power amplifier according to the prior art;

FIG. 3 shows a graph of achievable data rate in an EDGE system relatedto C/I;

FIG. 4 shows a flowchart detailing operation of an embodiment of theinvention; and

FIG. 5 shows a flowchart detailing operation of another embodiment ofthe invention.

The current GSM network is going to be used increasingly for data ratherthan voice calls in the near future. Indeed, it is estimated thateventually, data calls by users browsing the World Wide Web, forinstance, will eventually greatly outnumber voice calls over cellularnetworks.

This fact in conjunction with the increasing desire to use MCPAs createsnew problems in call-management. Specifically, more timeslots are likelyto be allocated to a single user, and the charging for use of thenetwork may well develop along the line of charges per byte of datatransferred, rather than connection time as is the norm at present.

The increasing use of GSM cellular networks for data traffic has animpact on power transmission requirements. This is particularly true inthe EDGE system. EDGE is a standard which supplements TDMA systems, suchas GSM, and allows relatively high speed data connections to beestablished using existing network infrastructure. The high speed isachieved via use of a different modulation scheme. GSM uses GaussianMinimum Shift Keying (GMSK) and EDGE uses 8-PSK. To boost the data ratefurther, several timeslots can be used by the same user for datatransfer. A drawback of EDGE is that its modulation scheme requires ahigher Carrier to Interference Ratio (C/I) than GSM in order to operatereliably. GSM typically requires a C/I of 9 dB, whereas EDGE needsapproximately 20 dB more than this.

Broadly speaking, there are two ways to increase C/I. The first way isto reduce the interference power (I). However, this is largely out ofthe control of the operator of the cellular system, as it is derivedfrom other systems, and indeed other signals within his system. Thesecond way is to increase the carrier power (C). However, this placesdemands on the transmission equipment, and goes on to cause interferenceto other systems, and so is not a viable option beyond a certain limit.

In EDGE systems, it has been found empirically that there is a directrelationship between achievable data rates and C/I. FIG. 3 shows a graphrepresenting empirical studies into the relationship. It can be seenthat higher data rates are available for higher C/I values. This is tobe expected, as a clearer signal mitigates the need for re-transmissionof data which is lost due to poor connection quality. However, it can beseen that the relationship is not linear, and that increasing C/Ilinearly does not cause the data rate to increase similarly. The maximumachievable data rate tails off at higher values of C/I.

If the MCPA is running near its maximum output power, and a new voice ordata call is initiated, or a handover is attempted into the cell it isserving, then a new timeslot on a carrier needs to be assigned. In thecase where a BTS is equipped with SCPAs, the only factor that needs tobe considered in determining whether a call can be accepted is whether aspare timeslot exists on any of the SCPAs. If a timeslot is available,the call is set up. In the case where an MCPA is used, there may well beone or more spare timeslots available, but now the factor which needs tobe considered is whether there is sufficient transmit power available tosupport the new call.

If the MCPA is supporting a given number of data and voice calls, andthe MCPA is operating at its maximum transmit power on all timeslots,one option is to refuse the new call set up. If the new call was anattempted handover, it might be transferred to another neighbouringcell. If that is not possible, it may stay connected to its currentcell. If that is not possible, then the call will be dropped. None ofthese scenarios is particularly desirable.

A preferable embodiment of the invention continuously monitors theallocation of timeslots, the nature of the call on each timeslot, andthe data rate being achieved for each data call. If an attempt is madeto set up a new call when the MCPA is operating at maximum transmitpower on all timeslots, or at such a level that insufficient transmitpower is available to support the new call, then there are severaloptions which can be pursued before call set up is refused.

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TRX0 −2 dB −8 dB −4 dB −10 dB −4 dB D:32V V V V TRX1 −12 dB  −12 dB  −6 dB   0 dB −4 dB V V D:30 V V TRX2 −4 DB−6 dB −2 dB −6 dB −12 dB V TRX3 −8 DB −8 dB  −6 dB −8 DB V V D:24 V TRX4−4 dB −2 dB −6 dB V D:26 V TRX5 −8 dB −8 dB −2 dB V V D:30 TRX6 −6 dB −8dB V V TRX7 −2 dB   0 dB −8 dB  −4 dB −10 dB  V V V V V

An example of a database that is maintained by the system for each MCPAis given above. It shows which timeslots are active on which TRX, whatthe power level associated with the call is, and whether the call isdata or voice. If it is a data call, the data speed is recorded also.‘V’ signifies a voice call, and ‘D’ signifies a data call, with thenumber following being the data rate in Kbit/s. Blank cells in the tablesignify no call activity on that carrier/timeslot pair.

Firstly, if one or more of the calls being supported by the MCPA is adata call, then in order to free up some transmission power to supportthe attempted new call, the transmission power associated with one ofthe data calls can be reduced by an amount equal to the amount needed tosupport the new call.

The drop in transmission power for the present data call will result ina drop in the achievable data rate, as shown in FIG. 3.

Preferably, if there is more than one data call in progress, then thedata call having the highest rate is reduced first.

Using the graph of FIG. 3, it can be seen that an EDGE call having adata rate of 32 Kbit/s can free up enough power for another call tooperate at a transmission power of −4 dB if its transmission power isdropped by a corresponding amount. Consequently, the data rateexperienced by that user drops to 26 Kbit/s. Such a drop in data ratewill have an effect on the user of the MS making that call, but it willnot be so dramatic as dropping the potential new call.

In the tabular example above, the data call on TRX0 at TS2 has a datarate of 32 Kbit/s and a transmit power level of −2 dB. As this call hasthe highest data rate, an embodiment of the invention would liberatesome power on TS2 by reducing the transmit power associated with TRX0.

In any event, under the EDGE standard, the data rate will drop as theuser experiences a weaker signal from the BTS. This means that at theedge of the cell, or in other areas of poor reception, the data rateachievable will be lower than if the MS were receiving a strongersignal. In the event that the signal reaching the MS is insufficient tomaintain an EDGE connection, the modulation scheme switches to that ofthe underlying network, for instance, GSM. This offers a lower datarate, but one that is robust and generally guaranteed. In the GSM case,using GMSK modulation, the data rate available is 9.6 Kbit/s.

The level to which trade-offs between data rate and call connectionoccurs is determined by the system operator on the basis of which eventis perceived to cause most inconvenience to users of the network. It isnormally preferable to maintain a call, even if the data rate isreduced, so that a new call can be accommodated, than to refuse a callset up or to drop a call because it is not possible to hand it over to anew cell.

FIG. 4 shows a flowchart which summarises operation of an embodiment ofthe invention.

At box 100, an attempt is made to set up a new call or hand over anexisting call from a neighbouring cell. This can be a voice or a datacall.

A check is made at 110 to determine whether sufficient transmit power isavailable in order to set up the call. This check is performed on thebasis of the contents of the database which records call activity for agiven MCPA.

If the call can be set up using the currently available free transmitpower capacity, then call set up occurs as shown at 120.

If there is insufficient transmit power capacity to set up the call, thenext check 130 determines whether a data call is listed amongst thecurrently supported calls.

If there is one or more data calls in progress then one of them willhave their transmission power reduced 140. In a preferred embodiment,the data call currently enjoying the highest data rate is selected tohave its transmission power reduced ahead of any other data calls. Inany event, the transmission power of any given data call will not bereduced below a guaranteed minimum.

Once this has been done, and sufficient transmission power has beenfreed up for the new call, the new call is set up 120.

If there are no data calls in progress, then it is necessary to refusecall set up of the new call 150.

Secondly, if, for example, two time slots have some spare transmit powercapacity, but not enough to support the new call individually, then thetimeslot allocations of the currently supported calls can be re-arrangedso that the two portions of spare capacity can be re-allocated on thesame timeslot so that the new call can be set up on that timeslot.

This will incur some delay in the call set up process while the existingtimeslot allocations are re-ordered, but this delay will not benoticeable to a user.

Thirdly, it may be possible in the future to attach priorities tocertain call types. This may be based on any number of different factorssuch as the charging tariff to which a customer belongs, whether a callis data, voice or fax, or whether a user has selected to pay acall-by-call premium to enjoy a high data rate. It is possible to assigncertain users the lowest possible priority so that they can effectivelyonly make emergency calls. If the calls being supported by the MCPAcontain a number of calls of differing priority levels, then thedecision on which call has its data rate reduced may be made based onthe relative priority levels of all currently supported calls.

This is summarised in FIG. 5, which shows a flowchart similar to that ofFIG. 4. All steps are identical except that step 140 is replaced by step240 whereby the data call having the lowest assigned priority has itstransmission power reduced before any other data call.

One or more of the embodiments herein described can be combined with oneor more of the other embodiments herein described.

These measures ensure that the maximum number of calls is maintained bythe communication network. The result may be reduced data rate for someusers in some circumstances, but it will allow more users access to thenetwork, which is generally preferable to calls being dropped athandover, or not set up in the first instance.

In the light of the foregoing description, it will be clear to theskilled man that various modifications may be made within the scope ofthe invention. In particular, the EDGE standard may be replaced withanother variable data rate transmission standard. The underlying networkhas been described in terms of GSM, but the skilled man will realisethat any suitable TDMA system could be used instead.

The present invention includes any novel feature or combination offeatures disclosed herein either explicitly or any generalisationthereof irrespective of whether or not it relates to the claimedinvention or mitigates any or all of the problems addressed.

1. A method of operating a Time Division Multiple Access (TDMA)communication system comprising a multi-carrier power amplifier,comprising the steps of: logging call activity on each timeslot of eachcarrier associated with the multi-carrier power amplifier; logging totaltransmit power of the multi-carrier power amplifier for each timeslot;and logging the data rate of any data calls being carried by themulti-carrier power amplifier, and wherein transmission power is madeavailable for a new call by reducing the transmission power of a datacall and a data call currently having a highest data rate has transmitpower thereof reduced before another data call having a lower data rate.2. A method as claimed in claim 1 wherein: the communication systemoperates according to the EDGE standard.
 3. A method as claimed in claim1, wherein: timeslots are assigned such that the total transmissionpower is substantially constant across all timeslots.
 4. A method asclaimed in claim 2, wherein: timeslots are assigned such that the totaltransmission power is substantially constant across all timeslots.
 5. Amethod as claimed in claim 1 wherein: each call has an associatedpriority level, and a call having a lower priority level has a transmitpower level thereof reduced before a call with a higher priority level.6. A method as claimed in claim 2 wherein: each call has an associatedpriority level, and a call having a lower priority level has a transmitpower level thereof reduced before a call with a higher priority level.7. A method as claimed in claim 3 wherein: each call has an associatedpriority level, and a call having a lower priority level has a transmitpower level thereof reduced before a call with a higher priority level.8. A method as claimed in claim 4 wherein: each call has an associatedpriority level, and a call having a lower priority level has a transmitpower level thereof reduced before a call with a higher priority level.9. A method as claimed in claim 1 wherein: if the multi-carrier poweramplifier is not operating at maximum transmit power, but insufficienttransmit power is available in any one timeslot, then timeslots arereallocated such that any available transmit power is re-allocated to asingle timeslot.
 10. A method as claimed in claim 2 wherein: if themulti-carrier power amplifier is not operating at maximum transmitpower, but insufficient transmit power is available in any one timeslot,then timeslots are reallocated such that any available transmit power isre-allocated to a single timeslot.
 11. A method as claimed in claim 3wherein: if the multi-carrier power amplifier is not operating atmaximum transmit power, but insufficient transmit power is available inany one timeslot, then timeslots are reallocated such that any availabletransmit power is re-allocated to a single timeslot.
 12. A method asclaimed in claim 5 wherein: if the multi-carrier power amplifier is notoperating at maximum transmit power, but insufficient transmit power isavailable in any one timeslot, then timeslots are reallocated such thatany available transmit power is re-allocated to a single timeslot.
 13. Amethod as claimed in claim 1, wherein: the transmit power associatedwith a data call has a defined minimum value.
 14. A method as claimed inclaim 2, wherein: the transmit power associated with a data call has adefined minimum value.
 15. A method as claimed in claim 3, wherein: thetransmit power associated with a data call has a defined minimum value.16. A method as claimed in claim 5, wherein: the transmit powerassociated with a data call has a defined minimum value.
 17. A method asclaimed in claim 9, wherein: the transmit power associated with a datacall has a defined minimum value.
 18. A method as claimed in claim 13,wherein: the defined minimum value is equivalent to the data rateachievable using a GMSK modulation scheme.
 19. A method as claimed inclaim 1, wherein: a new call will not be set up if insufficient transmitpower is available to support the new call.
 20. A method as claimed inclaim 2, wherein: a new call will not be set up if insufficient transmitpower is available to support the new call.
 21. A method as claimed inclaim 3, wherein: a new call will not be set up if insufficient transmitpower is available to support the new call.
 22. A method as claimed inclaim 5, wherein: a new call will not be set up if insufficient transmitpower is available to support the new call.
 23. A method as claimed inclaim 9, wherein: a new call will not be set up if insufficient transmitpower is available to support the new call.
 24. A method as claimed inclaim 13, wherein: a new call will not be set up if insufficienttransmit power is available to support the new call.
 25. A communicationsystem comprising: a multi-carrier power amplifier; and a databasecomprising information concerning call activity on each timeslot of eachcarrier associated with the multi-carrier power amplifier; informationconcerning total transmission power of the multi-carrier power amplifierfor each timeslot; and information concerning a data rate of any datacalls supported by the multi-carrier power amplifier; and wherein themulti-carrier power amplifier reduces the transmission power associatedwith a data call in order to make transmission power available for a newcall and a data call currently having a highest data rate has transmitpower thereof reduced before another data call having a lower data rate.